1. Field of the Invention
The present invention relates to an organic light emitting display device and a method of driving the same.
2. Description of Related Art
Recently, there have been various types of flat panel display devices with reduced weight and volume in comparison to cathode ray tube display devices. The flat panel display devices include a liquid crystal display device, a field emission display device, a plasma display panel, an organic light emitting display device, and the like.
Among these flat panel display devices, the organic light emitting display device displays images by using organic light emitting diodes (OLEDs) that emit light through recombination of electrons and holes. The organic light emitting display device has a fast response time and is capable of being driven with low power consumption.
FIG. 1 is a schematic circuit diagram showing a conventional pixel of an organic light emitting display device.
Referring to FIG. 1, the conventional pixel 4 of the organic light emitting display device includes an organic light emitting diode OLED and a pixel circuit 2 coupled to data a line Dm and a scan line Sn for controlling the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit 2, and a cathode electrode of the organic light emitting diode OLED is coupled to a second power source ELVSS. The organic light emitting diode OLED emits light with a luminance (e.g., a predetermined luminance), corresponding to a current supplied from the pixel circuit 2.
When a scan signal is supplied to the scan line Sn, the pixel circuit 2 controls an amount of current supplied to the organic light emitting diode OLED, corresponding to a data signal supplied through the data line Dm. To this end, the pixel circuit 2 includes a second transistor M2 coupled between a first power source ELVDD and the organic light emitting diode OLED; a first transistor M1 coupled to the second transistor M2, the data line Dm and the scan line Sn; and a storage capacitor Cst coupled between a gate electrode of the second transistor M2 and a first electrode of the second transistor M2.
A gate electrode of the first transistor M1 is coupled to the scan line Sn, and a first electrode of the first transistor M1 is coupled to the data line Dm. A second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst. Here, the first electrode is one of the source and drain electrodes, and the second electrode is the other one of the source and drain electrodes. For example, if the first electrode is a source electrode, the second electrode is a drain electrode. When a scan signal is supplied from the scan line Sn, the first transistor M1 coupled to the scan line Sn and the data line Dm is turned on to supply a data signal supplied from the data line Dm to the storage capacitor Cst. Accordingly, a voltage corresponding to the data signal is charged into the storage capacitor Cst.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst, and the first electrode of the second transistor M2 is coupled to the other terminal of the storage capacitor Cst and the first power source ELVDD. A second electrode of the second transistor M2 is coupled to the anode electrode of the organic light emitting diode OLED. The second transistor M2 controls an amount of current supplied to the second power source ELVSS via the organic light emitting diode OLED from the first power source ELVDD, corresponding to a voltage value stored in the storage capacitor Cst. Here, the organic light emitting diode OLED emits light corresponding to an amount of current supplied from the second transistor M2.
Typically, the conventional pixel 4 of the organic light emitting display device displays an image having a predetermined luminance by repeating the aforementioned process. In a digital driving method in which the second transistor M2 is operated as a switch, first power ELVDD and second power ELVSS are supplied to the organic light emitting diode OLED as they are, and accordingly, the organic light emitting diode OLED emits light by constant voltage driving. As such, in the digital driving method, the amount of current driving the organic light emitting diode OLED sensitively varies according to temperature variations and an increase in resistance caused by the degradation of the organic light emitting diode OLED. Therefore, an image having a desired luminance may not be displayed.
More specifically, an amount of current supplied from the pixel circuit 2 to the organic light emitting diode OLED is generally changed corresponding to the change of temperature. Therefore, the luminance of an image displayed may be changed corresponding to the change of temperature. Further, the organic light emitting diode OLED may be degraded as time elapses. When the organic light emitting diode OLED is degraded, its resistance increases. Accordingly, a current that flows into the degraded organic light emitting diode OLED is decreased corresponding to the same voltage difference across the organic light emitting diode OLED, and therefore the luminance may be lowered.